An intense acoustic scattering layer at the top of a mid‐ocean ridge hydrothermal plume

Abstract

We have used a tethered oceanic profiling package consisting of a conductivity‐temperature‐depth (CTD) sensor, transmissometer and acoustic Doppler current profiler (ADCP) to investigate the plume emanating from a known hydrothermal vent site at a depth of 2200 m near the crest of Endeavour Ridge in the northeast Pacific Ocean. Acoustic profiles obtained in August 1988 about 3 km to the southwest of the vent site revealed the presence of an intense scattering layer centered at approximately 1870 m depth near the top of the neutrally buoyant portion of the plume. Peak intensity anomalies in the 100‐m‐thick scattering layer ranged from 10 to 15 dB and were of comparable magnitude to upper ocean values observed in the euphotic zone. Below the top of the plume, backscatter intensities were significantly below background levels. We speculate that the strong backscatter layer consists of highly mobile macrozooplankton which congregate near the top of the plume to take advantage of the increased concentrations of nutrients originating with vertical buoyant transport of chemosynthetic bacteria and fine‐grain particles from the vent site. At the same time, the animals are able to avoid toxic levels of hydrothermally derived metals and sulfur inherent to the main body of the plume.

Diversity and abnormalities of cyclopoid copepods around hydrothermal vent fluids, Kueishantao Island, north-eastern Taiwan

“…These waters have a negative potential effect on the health of the organisms residing or passing through the vent fluid waters (Thomson et al 1991, Burd et al 1992. The effects of hydrothermal effluents are not confined to the sea-floor environment.…”

Investigating the Relationship Between Fin and Blue Whale Locations, Zooplankton Concentrations and Hydrothermal Venting on the Juan de Fuca Ridge

“…The backscatter intensity is a relative measure of the number of macrozooplankton within a given volume of water. Following Thomson et al [1991b], we standardized our acoustic data for inter-tow comparisons by subtracting the depth-averaged background value of the target strength for each of the four beams to yield a target strength anomaly, TS . By making reasonable assumptions about the relationship between the effective length scale of the scatterers and the instantaneous net biomass m k ' k , a relationship can be derived relating the total biomass of the tow, M, to the target strength anomaly.…”

The importance of hydrothermal venting to water-column secondary production in the northeast Pacific

“…The entire system was towed at an angle through the water column Thomson, 1993, 1994). Thomson et al (1991Thomson et al ( , 1992 describe the acoustic backscatter measurements, background calibration and use of other instrumentation.…”